1. Field of the Invention
The present invention relates to an image forming apparatus adapted for use as an electrophotographing apparatus, such as a copying machine, printer, etc.
2. Description of the Related Art
Conventionally, electronic copying machines and printing machines are known as apparatuses for copying the original. In general, the electronic copying machines are easy to operate, requiring only simple arrangements for the copying operation. In printing machines, on the other hand, a negative plate is prepared from the original, and the original image can be repeatedly transferred to recording sheets. Once the negative plate is obtained, therefore, the printing operation can be performed at high speed. The more copies taken of the same original, therefore, the lower the printing costs will be.
In conventional printing machines, however, the cost of the negative plate constitutes a large proportion of the total printing cost, unless about 1,000 or more copies are taken of the same original. Thus, the printing cost is higher than in the case of electronic copying machines. Since the preparation of the negative plate requires an additional process, printing work takes more time and labor. In contrast, the electronic copying machines must repeat processes for charging a photoreceptor drum, exposing the drum surface to an optical image of the original, developing a latent image, and discharging the drum, in taking each copy of one original. Even if the number of copies taken of the same original is increased, therefore, the cost for each copy cannot be lowered. Moreover, the copying speed is much lower than the printing speed of the printing machines.
To cope with these problems, improved electronic copying machines have been put to practical use. According to these copying machines, the copying speed is made substantially as high as the printing speed without changing the existing technological concept. Also, these novel machines do not require any such delicate operations or arrangements as printing machines require.
If the copying speed is further increased, however, very high electric power is required for the exposure of the optical image of the original and the fixation or charging of the developing agent. Accordingly, a regular commercial power supply (100V, 15A) cannot be utilized, and the developing speed must be made unduly high, at the sacrifice of the image quality.
Methods to solve these problems are disclosed in Japanese Patent Application Nos. 63-29273 and 63-29274, for example. In these methods, a first toner image corresponding to an original image is formed on a photoreceptor by the electrophotographing process, with use of a first magnetic toner, and the toner image is fixed on the photoreceptor. The fixed toner image is magnetized to form a magnetic latent image, which is used thereafter as a master image for magnetic printing. The magnetic latent image is developed by means of a second magnetic toner, and finally, a resulting second toner image is transferred to a recording medium. Once the magnetic latent image as the master image is formed, according to these methods, a large number of copies can be obtained at high speed by repeating the development by means of the second magnetic toner and the transfer of the toner image to the recording medium.
In apparatuses using the magnetic printing system described above, the magnetic toner conventionally used is of low coercivity, having the magnetic material content of 5 to 30% by weight. In general, however, the magnetic toner of this type cannot provide a sufficient magnetizing force, so that the residual magnetizing force of the master image for magnetic printing, using such a magnetic toner, is small. Thus, satisfactory magnetic printing cannot be effected, and the density of the resulting copy images is very low.